Muscarinic M3 receptors mediate total inositol phosphates accumulation in murine HSDM1C1 fibrosarcoma cells

Abstract

Muscarinic receptors in murine fibrosarcoma HSDM1C1 cells were characterized using both radioligand binding and total inositol phosphates accumulation (IPs). Muscarinic agonists elicited a concentration-dependent enhancement of IPs accumulation with a maximum of 14-fold stimulation above basal level. The following potencies (-log EC50) were observed for the full agonists: (+)-cis-dioxolane 5.4, oxotremorine-M 5.3, (+)-muscarine 5.2 and carbachol 5.0. Bethanechol (4.1) and arecoline (5.0) were partial agonists, evoking 43 and 55%, respectively of the maximum level of stimulation to (+)-cis-dioxolane, whereas pilocarpine and McN-A-343 were inactive as agonists (1 mumol/l-1 mmol/1). The apparent affinities for muscarinic antagonists (-log KB) estimated by Schild regression were: 4-DAMP (4-diphenylacetoxy-N-methylpiperidine methiodide) 9.2, dicyclomine 7.0, pirenzepine 6.9, (+/-)-p-F-HHSiD (para-fluoro-hexahydro-siladifenidol) 7.0, AF-DX 116 6.2, methoctramine 5.7. In saturation binding studies using [3H]N-methylscopolamine a homogeneous population of sites was identified, with a density of 145 pmol/mg protein. In competition radioligand binding studies, the following apparent affinities (-log Ki) were observed: 4-DAMP 9.7, dicyclomine 8.3, (+/-)-p-F-HHSiD 7.6, AF-DX 116 6.8, methoctramine 6.6 and gallamine 6.8. In binding studies all antagonists studied recognized a single population of sites, as judged by the Hill coefficients from the displacement isotherms. These data are consistent with HSDM1C1 cells expressing an apparent homogeneous muscarinic M3 population that mediates a large level of total IPs accumulation. This clonal line may provide a useful model to further elucidate relationship between endogenous muscarinic M3 receptor stimulation and IPs accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)